The invention relates to a method and apparatus for determining the amount of fuel in the tanks of a spacecraft (for example, a satellite) under weightless conditions during its mission. Any storable fuels that are present in the tanks in liquid form under nominal operating conditions (such as hydrazine, monomethyl hydrazine, dinitrogen tetroxide, xenon) may be considered.
Effective management of a spacecraft, such as a satellite, is essential, due to its high economic cost. Such control requires a precise prediction of the remaining lifetime of the spacecraft so that it is possible, for example, to use its remaining fuel to free up the orbital position for successor satellites at the end of the lifetime. One parameter for determining the remaining lifetime is the quantity of fuel that exists in the fuel tanks of the spacecraft. It is therefore important to be able to predict exactly the amount of fuel.
Various methods of measuring the amount of fuel are known. For example, the bookkeeping method computes fuel consumption based on number and the duration of all engine ignitions since the start. Because the consumption of the engines is known approximately, it can be estimated how much fuel is still present in the fuel tanks. Further, the PVT (pressure-volume-temperature) method is known. The latter uses the ideal gas law, in which the gas temperature and the gas pressure in the tank are used to calculate how much gas is still present. The remaining amount of fuel can be calculated therefrom.
A significant disadvantage of these methods, however, is that they are insufficiently precise to satisfy today's requirements.
Published U.S. Patent No. 2004/231413 discloses a further method of measuring the amount of fuel, which requires additional hardware in the spacecraft, and therefore increase its weight of the spacecraft. Furthermore, the additional hardware increases the cost of the spacecraft.
One object of the present invention is to provide a method and apparatus for determining the amount of fuel which is available in the tanks of a spacecraft with a higher precision than could be achieved by the known techniques.
This and other objects and advantages are achieved by the method and apparatus according to the invention, which includes an orifice, a flow latch valve for selecting an orifice to be used and a control valve for releasing a stream of a pressurizing gas from a high-pressure tank.
Furthermore, an apparatus is also provided according to the invention which comprises a check/latch valve for selecting the at least one fuel tank.
Furthermore, an apparatus is also provided according to the invention, in which the control valve is closed when a desired pressure is reached in a fuel tank that is to be pressurized.
In addition, an apparatus is provided according to the invention, in which the control valve is closed by telecommand.
Furthermore, according to the invention, an apparatus is provided, in which the orifice, the at least one flow latch valve and the at least one control valve are arranged and/or designed in a redundant manner.
According to the invention, a method is provided for measuring the amount of fuel aboard a spacecraft under weightless conditions, which method comprises the following steps: Opening of a flow latch valve for selecting an orifice to be used and releasing a stream of a pressurizing gas from a high-pressure tank via a control valve.
Furthermore, according to the invention, a method is provided which, in addition, comprises the step of opening a check/latch valve for selecting a fuel tank to be pressurized.
Furthermore, a method is provided according to the invention which, in addition, comprises the step of releasing a stream of a pressurizing gas from at least one high-pressure tank via a control valve.
Furthermore, a method is provided according to the invention which, in addition, comprises the step of closing the control valve when a desired pressure has been reached in a fuel tank.
Furthermore, a method is provided according to the invention which, in addition, comprises the step of measuring the pressure and temperature in both the high-pressure tank and the at least one fuel tank, before and after the filling operation.
Furthermore, a method is provided according to the invention which, in addition comprises the step of calculating the amount of fuel in the fuel tank selected for the pressurization.
Furthermore, a method is provided according to the invention, in which the amount of fuel is determined using a first pressure, a first temperature, a first vapor pressure, a time period, a second pressure, a second temperature, a second vapor pressure, a third pressure, a third temperature, a fuel tank volume as a function of the internal pressure, a temperature-dependent fuel density, an orifice coefficient and a parameter which takes into account the dependence of the critical flow on the supply pressure.
Furthermore, according to the invention, a method is provided which determines the amount of fuel by means of the formulas
      mProp    :=                                                                                                                                                          p                        ⁢                                                                                                  ⁢                        1                                            -                      pVI                                        ⁢                                                                                                                      T                    ⁢                                                                                  ⁢                    1                                                  ·                                  VT                  ⁡                                      (                                          p                      ⁢                                                                                          ⁢                      1                                        )                                                              -                                                                                                                                                                              p                        ⁢                                                                                                  ⁢                        2                                            -                                              pV                        ⁢                                                                                                  ⁢                        2                                                              )                                                        T                    ⁢                                                                                  ⁢                    2                                                  ·                                  VT                  ⁡                                      (                                          p                      ⁢                                                                                          ⁢                      2                                        )                                                              +                              delta_mHe                ·                R                                                                                                                    p                ⁢                                                                  ⁢                1                            -                              pV                ⁢                                                                  ⁢                1                                      ⁢                                                                      T            ⁢                                                  ⁢                          1              ·                              rho                ⁡                                  (                                      T                    ⁢                                                                                  ⁢                    1                                    )                                                                    -                                            p              ⁢                                                          ⁢              2                        -                          pV              ⁢                                                          ⁢              2                                            T            ⁢                                                  ⁢                          2              ·                              rho                ⁡                                  (                                      T                    ⁢                                                                                  ⁢                    2                                    )                                                                          with      delta_mHe    :=          dt      ·      K      ·              pHe        c            ·                        [                      1                          R              ·              THe              ·                              [                                                      (                                                                  a                        THe                                            -                      b                                        )                                    +                                      1                    pHe                                                  ]                                              ]                          1          2                      and                    VT        ⁡                  (          p          )                    :=                        V          ⁢                                          ⁢          0                +                  aV          ·          p                      ⁢                  and            rho      ⁡              (        T        )              :=          rho      ⁢                          ⁢              20        ·                              [                          1              +                              α                ⁢                                                                  ⁢                                  Prop                  ·                                      (                                          T                      ⁢                                              -                                            ⁢                      293                                        )                                                                        ]                    .                    in which the above variables are identified in the following Table:
Measured ValuesP1Fuel tank pressure before the fill-upP2Fuel tank pressure after the fill-upT1Fuel tank temperature be-fore the fill-upT2Fuel tank temperature after the fill-updtTime period of the fill-upoperationPHeAverage helium tank averaged over the fill-up pressureoperationTheAverage helium tem-averaged over the fill-up peratureoperationFuel TankVT(p)Fuel tank volume as a func-tion of the internal pressureV0Tank volume (at ambient the tank volume is modeled pressure) hereby as a function of the internal pressureaVApproximation coefficient The tank volume is modeled for the linear modeling of hereby as a function of the the tank volumeinternal pressure.FuelpV1p)Vapor pressure before the is determined as a function fill-upof the temperature by way of a not indicated equationpV2Vapor pressure after the is determined as a function of fill-upthe temperature by way of a not indicated equation.rho(T)Density of fuel as a func-tion of the temperaturerho20Density at 20° C.The fuel density is hereby modeled as a function of TaPropApproximation coefficient The fuel density is hereby for the linear modeling ofmodeled as a function of Tthe density OrificeKParameter, combining the determined by measuringorifice coefficient, the orifice function and the cross-sectional surfaceCParameter, taking into ac-determined by measuringcount the dependence of the critical flow onthe supply pressurePhysical constants of the Pressurizing GasRGas constant for example, in the case of helium R-2078 N * m/(kg * K)a, bEmpirically determinedfor example, in the case of compressibility coeffi-the allied formulation cientsa = 1.48E−6 K/Pa, b = 4E−10 1/Pa
The method and apparatus and methods according to the invention have the advantage that they permit the measuring and determining of the amount of fuel available in the spacecraft with a greater precision than can be achieved by means of the known methods.
It is a further advantage of the method and apparatus according to the invention that, as a result, the measuring and determination of the amount of fuel can be carried out several times during the mission duration of the spacecraft, without restricting the operation of the spacecraft.
Furthermore, the method and apparatus according to the invention have the advantage that they require only components of the spacecraft that are present anyhow for the operation of the spacecraft. Accordingly, the method and apparatus according to the invention neither increase the weight of the spacecraft nor raise its costs.
It is of course apparent that, in each of the foregoing embodiments, the method and apparatus according to the invention can be supplemented by using a plurality of each or all of the components which are mentioned above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.